Prior art infrared zoom lenses include a thermally insulated infrared zoom lens that has optical elements arranged in series from the foremost position closest to the object toward the focal point along the optical axis, namely, first to third lens elements in this sequence where the first lens element has its first and second major surfaces opposed to each other to exhibit positive magnification power, the second lens element has its first and second major surfaces opposed to each other to exhibit negative magnification power, and the third lens element has its first and second major surfaces opposed to each other to exhibit positive magnification power. The first and third lens elements are made of a first substance while the second lens element alone is made of a second substance different from the first substance, and a variation in refractive index of the first substance due to a variation in its temperature (dn/dT) is smaller than that of the second substance, and either one or both of the second major surfaces of the first and third lens elements is formed in diffractive surface (see Patent Document 1 listed below).
Another prior art infrared zoom lens has first to third groups of lens pieces arranged in series from the foremost position closest to the object, and during the zooming, the first and third lens groups are essentially fixed while the second lens group alone are movable where each of the first to third lens groups has at least one lens piece made of zinc sulfide (see Patent Document 2).
Still anther prior art infrared zoom lens is that which incorporates optics dedicated to infrared rays raging 3 to 5 μm or 8 to 12 μm in waveband and which has five groups of lens pieces arranged in series from the foremost position closest to the object, namely, a first lens group consisting of one or two lens pieces to exhibit positive power, a second lens group consisting of one or two lens pieces to exhibit negative power, a third lens group of a single negative meniscus lens having its concave surface positioned closer to the object, a fourth lens group of a single convex lens piece, and a fifth lens group consisting of at least four lens pieces where the rearmost lens piece closest to the imaging field is a positive meniscus lens having its convex major surface faced toward the object; and during the zooming, the first, fourth and fifth lens groups are essentially fixed while the second and third lens groups are movable so that displacing the second lens group along the optical axis permits magnification rate to alter, and meanwhile, displacing the third lens group along the optical axis enables to correct the imaging point under the requirements as defined in the following formulae:1.00<f1/ft f2/ft<−0.400.35<f5/ft<0.70where ft is a focal length of the entire optics at the telephoto end, f1 is the focal length of the first lens group, f2 is the focal length of the first lens group, and f5 is the focal length of the fifth lens group (see Patent Document 3).